In the following molecular drawing, I want to only circle the rightmost OH of the first molecule and the leftmost H of the second, to show that they are the ones that are involved in the formation of water:

\documentclass{article}
\usepackage{chemfig} % for typesetting molecules
\begin{document}
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-OH} % I want to circle this last OH
\chemsign{+}
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)} % I want to circle the H of the H_2N near the beginning
\chemrel{<>} \\
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-N(-[2]H)-CH(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)}
\chemsign{+}
\chemfig{H_2O} \\
\end{document}

I know that if I put

\setnodestyle{draw,circle}

on the very first line after \begin{document}, then I can make all the nodes (atoms) circled, but I only want to circle the two I mentioned, but I'm not having success inserting a \setnodestyle inside the \chemfig argument. Does anyone know how to custom modify just one node's style in chemfig?